Lactoferrin is known as an iron-binding protein distributed in tear, saliva, peripheral blood, milk and the like. It has been known that lactoferrin has various physiological activities, for example, antibacterial activity against harmful bacteria (B. J. Nonnecke, and K. L. Smith; Journal of Dairy Science; Vol. 67, p. 3; 1984), activity for promoting iron absorption by the intestine (G. B. Fransson et al; Journal of Pediatric Gastroenterology and Nutrition; Vol. 2, p. 693; 1983), anti-flammatory activity (J. V. Bannister et al; Biochimica et Biophysica Acta; Vol. 715, p. 116; 1982) and so on.
Therefore addition of lactoferrin to foods, processed foods, medicines, cosmetics and the like is desirable.
However, lactoferrin is unstable to heating at near neutral pH, and heat treatment of lactoferrin may result in denaturation of lactoferrin. It is reported that the physiological activities of lactoferrin are almost lost by heating at 62.5.degree. C. for 30 minutes, and complete denaturation is achieved by heating at 70.degree. C. for 15 minutes (J. E. Ford et al; Journal of Pediatrics, Vol. 90, page 29; 1977). In this connection, a typical condition of traditional thermal sterilization of milk is heating at 63.degree. C. for 30 minutes.
It is often necessary to heat foods, feeds, medicines and cosmetics for pasteurization, sterilization, or cooking, however, sufficient thermal treatment could not be applied to lactoferrin or matters containing lactoferrin as an ingredient for utilizing its physiological activities.
The inventors of the present invention have exerted their efforts to develop a method for heat treatment of lactoferrin or matters containing lactoferrin without losing its physiological activities, and have found that when lactoferrin or matters containing lactoferrin are heated under acidic conditions, its physiological activities such as antibacterial activity, iron-binding activity and antigenicity are scarcely affected. This invention is based on this discovery.